New Invocation - Critique Needed

[Dragongrief]

Hi All,

One of my players is working on a tiefling barbarian/warlock, inspired by Hellboy. He made two requests, but will work with what he has if I say no to either/both.

1) A permanent shield (the thick, hardened arm)
2) The ability to use the Mage Armor invocation with Con instead of Dex (essentially making it usable with Unarmored Defense)

So this is what I've come up with so far:

Arm of [insert dangerous sounding name]
Your right/left arm is oversized and covered in thick, armored skin. You are always considered to be wielding a shield, and your Dexterity bonus is considered to be +2 for purposes of Armor Class. You have Disadvantage on Dexterity Checks that require finesse with that hand.

It ends up giving a +4 total AC bonus, equivalent to having a shield and 14 Dex, but Disadvantage on specific Dex checks. Does this sound reasonable for an Invocation?

 

[Dragongrief]

Any takers on whether this is balanced, OP, or weak (double the last one).

The logic behind it was:
*He already has shield proficiency, so this, essentially, allows it to be permanently attached at the cost of disadvantage to fine manipulation.
*The second basically replaces his Unarmored Defense feature with AC 12+Con Mod. This allows using Dex as a dump stat (to the detriment of Dex saves, initiative, etc.), and bring his important abilities down to 3 scores.